1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine by which air-fuel ratio of air-fuel mixture to be inducted into the engine is controlled at target levels, and more particularly to the system for carrying out so-called cruising lean control in which the air-fuel ratio is changed from a stoichiometric target level to a lean target level in accordance with engine operating conditions.
2. Description of the Prior Art
Recently there has been a tendency of imposing higher requirements to automotive engines in which particularly achievement of exhaust emission improvement, high power output and high fuel economy is required at higher levels although they are conflicting with each other.
In view of this, so-called cruising lean control has been proposed and tried in which the air-fuel ratio of air-fuel mixture to be supplied to the engine is controlled at a lean value in a relatively low engine load operating condition particularly from the view point of fuel economy, although the air-fuel ratio is usually controlled at a stoichiometric value. An air-fuel ratio control system for carrying out such control is disclosed, for example, in Japanese Patent Provisional Publication No. 59-51147.
With this air-fuel ratio control system, a lean air-fuel ratio is selected in a predetermined operating region or condition determined from vehicle load and acceleration which are calculated in accordance with intake pipe pressure, changing rate of engine speed (or changing rate of vehicle speed) and throttle valve position, thereby improving fuel economy. In addition, learning control has been proposed from the view-point of improving exhaust emission, in which deviation between an air-fuel ratio correction coefficient and its target level in a stoichiometric air-fuel ratio control is memorized in accordance with output of an oxygen sensor, thereby accomplishing a feedback control of the air-fuel ratio correction coefficient into the target level as disclosed in Japanese Patent Provisional Publication No. 56-121842.
However, with such conventional air-fuel ratio control systems, the cruising lean control (or feedforward control) is carried out in a lean region in accordance with a learning coefficient obtained upon learning during the stoichiometric control (or feedback control into the stoichiometric value). Accordingly, a learning value in the lean region is usually not learned, thus lowering precision of the learning value in the lean region. This lowers the precision of air-fuel ratio control in the feedforward control thereby resulting in deteriorated exhaust emission and driveability. Such problems are particularly severe in the air-fuel ratio control system accomplishing the cruising lean control. Otherwise, such conventional air-fuel ratio control systems may be somewhat improved by using high precision sensors; however, such sensors are very expensive thereby inviting high production cost of the system.
Thus, with the conventional air-fuel ratio control system in which the cruising lean control and the learning control are merely combined, none of the feedback control of air-fuel ratio and the learning control is carried out in the lean region in which a lean air-fuel ratio is selected. Therefore, the conventional air-fuel ratio control systems have been required to be improved in precision of control.